A conventional syringe consists of a cylindrical barrel member, most commonly made of a thermoplastic material such as polypropylene. The conventional syringe typically has a reduced diameter distal end adapted to receive a needle thereon and an open proximal end adapted to receive a plunger rod assembly therein. The plunger rod assembly generally consists of an elongate plunger rod which is commonly made of a thermoplastic material such as polypropylene and a flexible plunger tip on the distal end thereof. The plunger tip is commonly made of an elastomeric material such as butyl or ethylene polypropylene rubber.
One of the functions of the plunger tip is to provide a substantially air tight seal with the interior surface of the syringe barrel so that movement of the plunger tip distally and proximally in the syringe barrel will cause medication, blood or other fluids to be drawn into or forced out of the distal end of the syringe barrel. The plunger tip is moved through the inside of the syringe barrel by applying axial force to the rigid plunger rod which has sufficient length to be accessible through the proximal end of the syringe barrel. Therefore, the preferred plunger tip is designed to have sufficient flexibility so that it will create a seal with the interior surface of the syringe barrel without creating excessive resistance to the movement of the plunger tip through the syringe barrel.
In order to ensure an air tight seal with the interior surface of the syringe barrel, plunger tips are typically manufactured with an outside diameter which is significantly larger than the inside diameter of the syringe barrel. The plunger tip is designed such that when the plunger tip is introduced into the syringe barrel, the plunger tip is compressed sufficiently to provide adequate pressure between the barrel and the plunger tip to seal this interface. As a result of the plunger tip design, the interface of the plunger tip and the barrel maintains, at all times, a sealing pressure sufficient to withstand the challenges of filling and injecting even though the same magnitude of sealing pressure may not be required when the syringe is not in use. A major drawback to this approach is that the increased diameter of the plunger tip increases the frictional resistance to movement of the plunger tip in the barrel.
U.S. Pat. Nos. 4,500,310 and 4,543,093 granted to Christinger disclose a variable sealing pressure plunger rod assembly which utilizes a thermoplastic plunger tip. As disclosed in these patents, the distal end of the plunger rod is modified to include one or more tapered surfaces thereon which are adapted to contact one or more similarly tapered surfaces on the interior surface of a thermoplastic plunger tip. When the plunger rod of the disclosed devices is moved distally in the barrel, the tapered distal surface of the plunger rod contacts the tapered distal surface on the interior surface of the plunger tip to create a force component which is directed substantially outwardly from the interface of the respective surfaces. As a result of the force component created by the contact between the respective tapered surfaces, the radial diameter of the plunger tip adjacent to a rib on the outer surface of the plunger tip, temporarily increases to increase the sealing pressure between the plunger tip and the interior surface of the barrel. These patents disclose further embodiments wherein the plunger rod and thermoplastic plunger tip have similarly tapered proximal surfaces such that the outer diameter of the plunger tip adjacent to a proximal rib is temporarily increased as the plunger rod assembly is withdrawn proximally in the syringe barrel.
One of the potential problems with the designs disclosed in the Christinger patents is that the tolerances between the dimensions of the interior of the plunger tip and distal end of the plunger rod are very important to the efficient operation of the disclosed syringe. The critical dimensions required for proper operation of these syringes may be difficult to maintain with the quantity of syringes that are typically produced by a manufacturer. If these critical dimensions are not maintained, the tapered surfaces of the plunger rod and plunger rod may not be properly aligned when the syringe is assembled. If the tapered surfaces are not properly aligned, the plunger tip may pull off the distal end of the plunger rod during aspiration of the syringe or the desired expansion of the plunger tip may not be achieved, thereby creating the possibility that a proper seal will not be formed between the plunger tip and the syringe barrel. Finally, it is also possible that the distal end of the plunger rod may become wedged within the interior of the plunger tip so that the radial expansion of the plunger tip is maintained when the plunger rod is moved in the opposite direction. If this were to occur, the frictional resistance to further movement of the plunger rod through the syringe barrel would be greatly increased.
A further potential problem with the designs disclosed in Christinger patents occurs when the plunger rod is canted slightly in the syringe barrel. If the user applies a pressure to the plunger rod which is not directed fully along the longitudinal axis of the syringe barrel, the expansion of the plunger tip along the interior surface of the syringe barrel may not be applied equally along the interior surface of the syringe barrel and the plunger tip may leak. This potential problem is further magnified if the tapered surfaces of the plunger rod and plunger tip are misaligned.
U.S. Pat. No. 2,895,773 granted to McConnaughey discloses a variable diameter tensed ring piston. As disclosed in this patent, a piston cap is stretched over a plunger head on a plunger rod such that the piston cap is longitudinally stretched between a pair of rings located on the external surface of the plunger cap. According to this patent, the longitudinal stretching of the plunger cap between the rings tends to bring the rings back towards their normal position in relation to each other, thereby providing a tighter and more positive seal when the plunger rod is in a static position than when the plunger rod is moved distally or proximally in the syringe barrel.
The present invention is intended to provide a plunger rod assembly and syringe which may be readily mass produced and which will expand the diameter of the distal or proximal portion of the plunger tip consistently and reliably upon the application of distally or proximally directed forces thereto by the plunger rod to provide an efficient seal between the plunger tip and the syringe barrel while minimizing the force necessary to move the plunger rod assembly through the syringe barrel.